Motor control system



Feb. 18, 1941. MONTGOMERY I 2,232,073-

MQTOR CONTROL SYSTEM Original Filed Nov. 26; 1934 Jnnentor (Ittorneg Patented Feb. 18, 1941 UNITED STATES PATENT OFFICE MOTOR CONTROL SYSTEM Terryl B. Montgomery,

W auwatosa, Wls., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a'corporation of Delaware 3 Claims.

This invention relates in general to control systems and particularly to systems for controlling the dynamo electric machines used for tensloning materials being wound upon and unwound from reels or similar winding and unwinding mechanisms.

In systems of the above type, it is desirable to maintain a constant predetermined tension on the reeled material. Dynamo electric machines have been mechanically coupled to the reels and utilized to tension the material being reeled by energizing the machines so that one runs as a motor tending to wind the material on the winding reel and the other runs as a generator supplying a torque tending to prevent unwinding of the "unwinding .reel. Due to the change in eifectlve reel diameters during winding and unwinding of the material, the tensioning torque supplied by the dynamo electric machinm must also change in proportion in order to maintain the tension on the material constant.

Various attempts to provide suitable tensioning systems have utilized mechanical mechanisms actuated by the slack or tension in the material to control the dynamo electric machines. Such mechanisms have been found to be either so delicate as to easily get out of order or if made sufficiently rugged are not sufficiently accurate and stable to maintain the tension at the desired con- 30 stant value. When devices such as rollers in contact with the material are used, these devices are physically in the way and are also undesirable due to the fact that they leave a mark on the strip of material. If the back tension is supplied 3.; by hydraulic brakes, the tension will only be constant if the reel diameter remains constant, however, it is difilcult to secure constant friction with such mechanical devices. Furthermore, the energy required to produce this back tension is lost l in friction.

To maintain a given setting of back tension automatically throughout a pass of the material is very desirable as it reduces the amount of supervision necessary. The ease of control and -13 smoothness in operation which can be obtained make the electrical method of tensioning especially adaptable and in addition a substantial power saving is effected. A further advantage of the electrical power tension control is that con- J' stant tension may be maintained both at standstill and throughout any pass in either direction. I

It is, therefore, an object of this invention to provide an improved method of and means for 57 electrically and automatically maintaining a constant tension on material being reeled and unreeled.

It is also an object of the present invention to provide a tensioning system of the above type wherein the direction of the material is easily reversed and tension is automatically maintained after such reversal.

It is also an object of this invention to provide a tensioning system of the above type wherein the same means for maintaining tension while the material is being reeled and unreeled is utilized for maintaining tension while the material is stopped.

It is also an object of this invention to provide a tensioning system wherein a constant tension is maintained regardless of changes in lineal speed of the material.

It is also an object of the present invention to provide in a tensioning system of the type wherein dynamo electric machines are coupled to reels and energized to tension the material being reeled, a means responsive to breakage of the material for deenergizing the tensioning system and quicklystopping said machines.

It 'is a further object of the present invention to provide a strip rolling mill having a tensioning'system of the above type with an easy and convenient means for utilizing the tensioning dynamo electric machines in setting up the material in the mill.

These and other objects and advantages are attained, by this invention, various novel features of which will be apparent from the description and drawing herein and will be more particularly pointed out in the claims.

An illustrated example of an application of this invention is shown in the accompanying drawing in which the single figure is a diagrammatic showing of a control system embodying one form of the invention as applied to a strip rolling mill.

Referring to the drawing, a strip of material [2 is shown as being unwound from a reel I3, acted upon by rolls l and H and then wound upon reel M. The strip I2 is pulled through the rolls by driving the forward reel, which is shown as H, by connecting it to the drive shaft of the main drive motor 1 by a clutch 29. The motor I is energized from a generator 6 driven by a synchronous motor 5, the control for the drive being a Ward Leonard control of the fields 20 and 9 of the generator and motor. Also driven by the motor is an auxiliary generator 8 which supplies armature current to the back tension circuit.

Connected to the reels l3 and 14 respectively and the rolls Ill.

are the tensioning dynamo electric machines I and 2, the armatures of which are connected in series across the auxiliary generator 8. The field windings of the machines I and 2 are so connected that the machines tend to run in opposite directions, the machine 2 running as a motor tending to pull the strip I 2 in the forward or arrow direction and the machine I tending to pull the strip in the opposite direction, thus supplying a torque to the reel l3 providing the desired back tension. The machine i has a main field winding it which is separately excited from a small exciter 3 and a compensating field winding l5. Field winding I8 of machine 2 is similarly excited from the exciter 4. The control of the excitation of machines I and 2 to maintain a constant tension is efiected by controlling the energization of field windings 22 and 23 of the exciters 3 and 4 respectively. This is an advantage in that the currents involved are much smaller than they would be if the machines I and 2 were controlled directly.

In order to maintain a constant back tension, it is necessary to vary the excitation of the tensioning machines in direct proportion to the change in reel diameter while holding the armature current constant through the tensioning machines. The armature current is held constant by a regulator generally designated as 24 which is responsive to the current in the tension circuit and acts upon the field excitation of the leading machine. The flux of the trailing machine is varied in proportion to the change in reel diameter by a voltage balance regulator generally designated as 25.

The regulator 26 is controlled by balancing the generated voltage of the trailing machine against the voltage of a small generator 30 which is operated so that its voltage is a measure of the lineal speed of the strip between the trailing reel The generated voltage of the trailing machine is measured by adding to the voltage across the brushes, the voltage across one-half of the resistor 38' connected across the terminals of the awrili-ary generator 8. The gem erator 8 supplies the losse in the machines I and 2 and the voltage of the generator 8 is a measure of the IR drop in the circuit. Neglecting the resistance of the leads, the IR drop in the circuit is substantially twice the IR drop in each machine and these voltage drops are equal. Hence the voltage across one-half of the resistor 38 is a measure of the IR drop in one machine. As the terminal voltage of the trailing machine is the actual generated voltage less the IR drop through the machine, the addition of the voltage of one-half of the resistor 38 to the terminal voltage of the trailing machine I gives an accurate measure of the generated voltage of machine l. Since the actual generated voltage is directly proportional to the field flux, an accurate method of controlling the flux is obtained, for the regulator 26 operates to raise or lower the field current of exciter 3 thereby changing the excitation of machine I proportionately to the change in lineal strip speed which is a measure of the change in reel diameter.

The control of the main drive motor I is effected by controlling the field 20 of the generator 5 from zero to full generator voltage and for higher speeds by weakening the field 9 of the motor "I. This control is effected by a rheostat generally designated as having a generator field resistor 39 and a motor field resistor 40. A single operating member 41 controls both generator and motor fields and simultaneously controls a resistor 31 for a purpose that will be explained later. When the operating member 41 is at the left as viewed in the drawing, the mill is at its minimum speed, that is, the curent in motor field 9 is a maximum and the current in the generator field 20 is a minimum. When the member 47 is in the position shown, full generator voltage is applied and the motor field 9 is weakened. Further movement of the member 41 to the right further increases the mill speed.

An approximately constant torque, that is, a constant pulling tension is required to pull the strip through the mill. Therefore, at the beginning of any pass the main drive motor is connected to the empty leading reel and to start the mill the voltage of the generator 6 is raised by the rheostat 35 and the current in the motor I is increased until a torque suflicient to start the strip is obtained. For any further increase in the voltage of the generator 6 the current in the motor 1 remains substantially constant for a given reel diameter as the motor field strength is constant up to full generator voltage. With this further increase in generator voltage after the strip starts, since the motor field strength remains constant, the horsepower input is increased in direct ratio to the voltage and the speed of the motor increases in direct proportion. The drive is therefore a constant torque drive from zero speed to a given speed corresponding to 100% generator voltage.

Above said given speed the field current of the motor I is weakened and since the mill requires substantially constant torque to provide a constant pullin tension on the strip, the motor armature current increases approximately in proportion to the decrease in field flux and since the voltage of generator 6 is now constant the horsepower and consequently the speed of the motor increase in direct proportion to the increase in armature current. At any speed of the motor I (1) and (2) T=KI where T=torque at one foot radius, I=the armature current in amperes, :field fiux and K: a constant.

From zero speed to the speed corresponding to full generator voltage, I and 5 remain constant. From this given speed to full speed, I increases but decreases a like amount so that their product remains constant. These conditions obtain only for the instant that the mill is started and brought up to speed, that is, these conditions are correct only for one given reel diameter.

Considering actual conditions when the mill is started the power is applied to the empty leading reel which has no strip on it, but as the strip is pulled through the mill it winds up on the leading reel causing an increase in the efiective reel diameter of reel l4. With this increase in reel diameter the torque of the motor 1 must increase proportionally in order to effect the same pull on the strip, thus Horsepower: T X RPM X K where P is the necessary forward pull on the strip, T is the motor torque at one foot radius and D is the reel diameter in feet.

Since P is constant, the motor torque must increase in direct proportion to the reel diameter and from zero to a given speed .(full generator voltage), and since the motor field flux .is constant, the armature current of the motor I will increase in direct proportion to increase in reel diameter. Above this given speed the motor armature current increases in proportion to the decrease in motor field strength as Well as increase in reel diameter.

or the motor armature current increases directly as the reel diameter and inversely as the motor field strength.

After the strip is started through the rolls at any given speed setting, as the diameter of the diameter of the leading reel increases, the motor armature current will increase proportionally to supply the necessary torque. Therefore, the strip speed will increase in proportion to the increase in effective reel diameter, the motor speed remaining the same,

It will be seen from Equation 4 that a constant back tension pull P will be exerted by the trailing back tension machine if the armature current I is held constant and the field flux o is changed in proportion to the change in effective reel diameter D.

To facilitate an understanding of the back tension system, it is first considered for the hypothetical case of constant reel diameter, and with the mill at standstill, the auxiliary generator 8 will have a constant excitation and will gen erate a given voltage and current in the back tension circuit. This current causes the machines I and 2 to exert a given torque and tension on the strip I2. Assuming constant and equal field excitations on the machines I and 2 for the hypothetical case of constant reel diameter, if the reel is started in the forward direction, machine I will generate a voltage which will add to the voltage of the generator 8 and machine 2 will generate a counter-E. M. F. which will oppose these voltages. The voltages of machine I and machine 2 will be directly proportional to the speed and therefore at any mill speed the counter- E. M. F. of machine 2 will for practical purposes counter-balance the generated E. M. F. of machine I and a constant current will be-maintained in the back tension circuit, but the voltage will be proportional to the speed.

Under actual conditions of mill operation the conditions given above for constant reel diameter will be altered because as the strip unwinds from the trailing reel, the decreased effective diameter raises the speed of machine I. This would cause an increase in voltage and current in the back tension circuit and a consequent increase in torque whereas according to Equation 4 the torque should be reduced in proportion to the increase in reel diameter.

To obtain the correct torque and diameter relationship the constant current regulator 24 which is responsive to the current in the back tension circuit operates on the field current of the leading machine 2. If the current tends to increase above the predetermined setting of this regulator, it acts to increase the excitation and therefore the counter-E. M. F. of the leading machine 2, which will reduce the current. Similarly, if the current falls below the regulator setting, the excitation of machine 2 will be decreased to cause an increase in the current in the circuit. This maintains the current constant regardless against each other.

of resistance change in the circuit due to any heating of .the conductors.

The voltage balance regulator 26 has two control :coils 33 and 32 of equal strength balanced The coil 33 is connected across the armature of the machine I and oneha'lfcf the resistance 38 shunted across the terminals of the generator 8 through resistor 31 adjusted by the position of rheostat 35, and the manually adjusted resistor 45. This coil as previously disclosed, measures the generated voltage of the trailing machine I. The coil 32 is connected through the resistance 31 of the rheostat 35 to the pilot generator 3!! which is driven by the strip I2 and generates a voltage proportional to the lineal speed of the strip independently of the reduction due to rolls It].

For any given speed setting of the main motor I, the speed thereof will remain constant and the motor armature current will adjust itself to give the required torque and on any given pass with constant speed of the motor I, the strip speed Will increase in proportion to the increase in diameter of the forward reel I4. The strip speed is measured by the pilot generator 38 and the increase in voltage due to increase in strip speed operates on the voltage balance regulator .25 to raise the voltage of the trailing machine I to increase its horsepower input in proportion to the increase in speed of the strip. Also, this increase in strip speed during each pass increases the speed .of the trailing machine I and in addition this machine increases further in speed due to the decrease in diameter of the reel 13. Thus the speed of the trailing reel I3 and machine I must increase inversely as the square of the reel diameter, that is Where K is a constant and D is the effective diameter of the trailing reel, but from Equation 4 it is seen that to maintain constant tension, must vary directly as D, I remaining constant. Therefore since the generated voltage of the trailing machine I will tend to increase as D and is a measure of D and the voltage of the pilot generator 30 increases as D and is a measure of D, the voltage balance regulator 26 will decrease the field excitation of machine I in direct proportion to the reel diameter D because it will automatically maintain the generated voltage of the trailing machine I equal or proportional to the voltage of pilot generator 30, thus maintaining constant back tension throughout the pass.

To consider the operation of the system shown the strip I2 will be considered to be Wound on the reel I3, the reel I 4 being empty. To enter the end of the strip I2 into a slot in the reel I4, inching is obtained in the following manner: inching push button 5| is momentarily closed, closing a circuit for the inching contactor 162 through centact 15 of ICZ and contact 55 of the low voltage relay LVI and the normally closed contact of the overload relay 0L3. Relay l'CI is therefore energized only if the relays I02, LVI and 0L3 are deenergized. Closing of contact 61 connects a short circuit across machine 2. Closing of contact 68 energizes the relay BF to close the circuit to the auxiliary generator field I9, thus energizing machine I. A back contact 63 locks out relay 1C2, thus preventing inching in the opposite direction. Contact I3 closes a circuit for the dynamic breaking contactor TCI which disconnects the dynamic breaking resistor 43 from across the terminals of machine I by opening contacts 8!, and completes the armature circuit of machine I by closing contacts 83. Another contact H opens a short circuit around the main coil 56 of the timing relay BTT allowing this relay to close immediately energizing field contactors PF3 and PF4, closing the circuit of the field 22 of the exciter 3. The machine I will rotate only as long as the push button 5| is held closed.

The rheostat 2I controlling the field current of the auxiliary generator 8 is of the potentiometer type and when in midposition (as shown) no current flows through the field winding I9. When the rheostat is in the extreme left hand position (as viewed in the drawing), the voltage is a maximum in one direction and when in the extreme right hand position, the voltage is a maximum in the opposite direction. Therefore both the direction and speed of the inching is easily controlled.

When push button 5| is released contactors ICI and TCI will immediately open, the latter connecting the dynamic breaking resistance 43 to machine I, thereby causing it to come to rest under dynamic breaking. When ICI opens, a short circuit is established by contact II around the main coil 56 of the timing relay BTT so that after a predetermined time delay, if no further inching is done, field contactor PF3 will be opened. This feature prevents full field current being applied to the machine I continuously for a very long period which might cause overheating at stand-still.

When the desired inching has been effected and the strip is entered in the reel I4 the main motor control push button 55 will be closed energizing the main control. The back tension master switch 53 will be closed which energizes the relay LVI which closes its holding circuit through contact 62. Contact 65 of LVI locks out inching contactors ICI and IC2, thereby preventing further inching. Contact 64 closes the circuit for the relay BF, thus energizing the field I9 of the generator 8. Contact 63 closes the circuit for the relay LVX which by closure of contacts 9 and 99 energizes relays TCI and TCZ to disconnect dynamic breaking resistors 43 and M from the machines I and 2, respectively, and to complete the armature circuits of these machines at contacts 80 and I8. Contact 66 opens the short circuit around the main coil 56 of timing relay BTT allowing this relay to close and energize field contactors PF3 and PF4 which establish field current in the machines I and 2 by energizing fields 22 and 23 of exciters 3 and 4 through resistor 45 and contacts 85, 89, and 8B, 34. These machines exert a torque which supplies tension to the strip at standstill.

A control lever on the mill is now operated to close the switch FWD and engage the main motor drive to the reel l4 by clutch 29. The closing of the switch FWD energizes the relay FT3-4 and also the relay VT3. The relay VT3 by contact 9| connects the coil 33 of the voltage balance regulator 26 across the machine I and one-half of the resistance 38 through the resistance 31 of the rheostat 35. By contact 92 of VT3, the coil 32 of voltage balance regulator 26 is connected across the terminals of the pilot generator 30 through the resistance 31 of the rheostat 35. Energization of relay FT3-4 opens contacts 84 and 85, thereby deenergizing the fields of exciters 3 and 4 as above stated and energizing field winding 22 of exciter 3 through resistance 36 of the motor operated rheostat 25 and contact 82. Contact 83 closes the circuit for the field of exciter 4 through constant current regulator 24 and the contact of relay PF4.

As soon as the pass is started and the motors begin rotating, the constant current regulator 24 regulates the excitation of machine 2 and by controlling its counter-E. M. F. holds the current constant in the back tension armature loop circuit. The voltage balance regulator 26 operates to hold the voltage of the trailing machine I equal to the voltage of the pilot generator 30, thus as the speed of the pilot generator 30 increases (this machine has constant excitation) its voltage will increase in direct proportion. The voltage of the trailing machine I will increase accordingly, raising the horsepower to maintain a constant torque. However, as the trailing machine I increases in speed due to decrease in reel diameter of the reel I3, its speed will increase above the corresponding speed of generator 30 and its voltage will tend to increase above that of generator 30. The coil 33 of the regulator 26 will overbalance the coil 32 and close contacts 48, thereby energizing relay L.

Energization of relay L opens contact 95 and closes contact 98 to connect the motor 21 of the motor operated rheostat 25 across the direct current bus bars I00 so that it will rotate in a direction to move the rheostat contacts to the left as viewed in the drawing. This will increase the amount of resistance 36 included in the field circuit, thereby decreasing the field current in the field 22 and decreasing the flux in the machine I. As this decrease in flux will be in direct proportion to the decrease in diameter of the reel I3 and as the armature current is maintained constant, it will be seen from Equation 4 that the back tension imparted to the strip by machine I will be held constant. The amount of tension is controlled by controlling resistor 45 in the circuit of coil 33.

As the voltage of the back tension circuit varies directly as the speed of the main drive motor I varies, the coils 32 and 33 of the voltage balance regulator 26 must operate over a wide range of voltages from zero to a maximum which may be as high as 250 volts. It is not feasible to construct coils which will be sufiiciently sensitive at low voltages and yet sufiiciently rugged to withstand the maximum voltages without damage. This problem is solved by the present invention in the following novel manner. The coils 32 and 33 are wound for a low voltage so as to give accurate regulation at low mill speeds. To prevent damage to the coils at high speeds and yet maintain accurateness of response, the resistance 3'! of the rheostat 35 is connected in series with each of these coils. Thus as the rheostat 35 is operated to increase the mill speed and hence the voltages applied to coils 32 and 33, the

amount of resistance 31 in series with each of the coils is proportionately increased. The voltage drop across the resistance 31 increases proportionally to the mill speed and hence the sensitivity of coils 32 and 33 remains substantially constant. Therefore, although the voltage across coil 32 varies relative to the voltage across coil 33, whereby the regulator acts to maintain constant tension, the rheostat 35 maintains these voltages within predetermined relatively narrow limits, thus permitting the sensitivity of regulater 26 to remain substantially constant.

As an example, at full mill speed the voltages across the trailing machine I and pilot generator 30 may be about 250 volts, that is, varying about 1% or from 247.5 to 252.5. The voltage across coils 32 and 33, if 30 volt coils, will then also vary 1% or from 29.7 to 30.3. If the mill speed is then changed to half speed, the voltages across machines and 30 will be about 125 volts and will vary from 123.75 to 126.25 while the voltages across coils 32 and 33 remain between 29.7 and 30.3. Thus we have a means, operably dependent upon the means for varying the voltage of machines and 30 (which may vary between relatively wide limits), for maintaining the voltages across coils 32 and 33 between relatively narrow limits, and for maintaining the sensitivity of these coils substantially constant.

When the material has all passed through the rolls It) the mill will be brought to a stop by control of the main motor. The back tension system will continue to exert its torque since current is still supplied by generator 8 and the excitation remains on the machines I and 2. To start the next pass the control lever is thrown to reverse position closing the switch REV and connecting the main drive to reel I3 by means of clutch 28. This deenergizes relays VT3 and FT34 and energizes relays VTI and F'I|-2. The deenergization of relays VT3 and FT3-4 disconnects the connections as previously made and the energization of relay VTI connects the machine 2 and pilot generator 3| to the voltage balance regulator 26 through contacts 93 and 94 respectively. Energization of relay FTI-Z connects the field 22 of exciter 3 through the constant current regulator 24 through its contact 86 and through a contact 81 connects the field 23 of exciter 4 through the voltage balance regulator. Thus the machine and reel |3 become the leading machine and reel and the machine 2 and reel l4 become the trailing machine and reel and a constant tension is maintained on the strip by the machine 2 in a manner similar to that described for the first pass.

The above description covers the normal operation of the mill. However, on occasion of strip breakage, the forward or leading machine will rotate in the same direction as the main motor and the trailing machine will rotate in the opposite direction. It is, therefore, necessary to bring the back tension system to a standstill, as well as the main drive, as quickly as possible in order to prevent damage to the strip as well as to the equipment. Photoelectric relays 43 and 44 are so positioned relative to the strip |2 that the strip intercepts a beam of light necessary to energize these relays. Upon breakage of the strip or if for any other reason the beam of light is allowed to energize the relays 43 and 44, the contacts thereof are opened thereby deenergizing the relay PR which deenergizes relay LV|. This dcenergizes the back tension system and causes the machines and 2 to quickly come to a stop by means of dynamic braking. Further or alternative protection is obtained by means of friction relays 4| and 42. These relays are operated in dependence upon the direction of the strip, relay 4| being open as shown with the strip going in the forward or arrow direction. Upon breakage of the strip and rotation of the machine in the reverse direction, relay 4| closes its contacts, closing a circuit to the relay SF which opens the circuit of LVI similarly deenergizing the back tension system. The friction relay 42 operates similarly when the machine 2 is the trailing machine to energize the relay SR and deenergize the back tension system. A manual stop control is provided by switch 54 which deenergizes the holding circuit for relay LVI.

This case is a division of application Serial No. 754,769, filed November 26, 1934, now Patent No. 2,143,357, granted January 10, 1939.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. An electroresponsive relay comprising a pair of cooperating contacts, means comprising a plurality of balanced and opposing electroresponsive coils for controlling the operation of said contacts, means for energizing each of said coils from separate varying voltage sources, means for varying said voltages over a relatively wide range and means operably dependent upon the operation of said voltage varying means for maintaining the voltages across said coils within a predetermined relatively narrow range.

2. In combination, a first circuit, a second circuit, a relay provided with a first winding and a second winding, means connecting said first winding to said first circuit, means connecting said second winding to said second circuit, control means in circuit with said windings for maintaining the sensitivity thereof substantially constant, and means for varying an electrical condition of said first and second circuits and for operating said control means.

3. In a control system, a plurality of sources of varying voltages, a regulator comprising a plurality of coils operably balanced against each other, means for connecting one of said sources to one of said coils, means for connecting another of said sources to an opposing coil, a resistance in circuit with both said coils and connected between said coils and said sources, and means for simultaneously varying said voltages and said resistance in the same proportion.

TERRYL B. MONTGOMERY. 

